Variable interest entities - Unconsolidated VIE (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Variable interest entities
Other long-term investments	$ 355.5	$ 346.8
Total assets	6,757.6	6,413.8
Unconsolidated VIE
Variable interest entities
Other long-term investments	250.7	257.8
Total assets	250.7	257.8
On-Balance Sheet
Variable interest entities
Other long term investments	103.0	102.6
Total assets	103.0	102.6
Off-Balance Sheet
Variable interest entities
Other long term investments	8.8	16.3
Total assets	8.8	16.3
Other long-term investments
Variable interest entities
Other long term investments	111.8	118.9
Total assets	$ 111.8	$ 118.9